In recent years, the saving energy has become desirable from the point of view of environmental operations. In devices such as a cell phone, a digital camera and the like wherein a battery is used, reducing the electric power consumed in the devices has become more important from the view of increasing the service life of the battery. For this purpose, as for the power supply circuit, non-insulated step-down type switching regulators with high efficiency and reduced size in which an inductor is used are in wide use (hereinafter, referred to as switching regulators). The switching regulators are highly efficient at rated loads. However, since the consumption of electric current by the switching regulators themselves is comparatively large, the efficiency of the devices becomes extremely low when the devices are in a light-load drive mode such as a stand-by mode, a sleep mode and the like.
In order to improve the efficiency even if the devices are in the light-load drive mode, Japanese Laid-Open Patent Application No. 2002-300774 provides a method for reducing the electric power consumed in the switching regulator by switching PWM control to PFM control in the light-load drive mode so as to lower the switching frequency.
FIG. 5 is a schematic circuit diagram illustrating an example of the switching regulator.
In FIG. 5, a switching regulator 100 includes a PWM control circuit 101 and a PFM control circuit 102. The switching regulator 100 further includes a switching element 103 driven in the PWM control circuit 101 and another switching element 104 driven in the PFM control circuit 102.
In the normal operating mode, the PFM control circuit 102 stops operations. The PWM control circuit 101 operates so as to control switching on and off the switching element 103. In the light-load drive mode, the PWM control circuit 101 stops operating. The PFM control circuit 102 operates so as to control switching on and off the switching element 104.
Since a large amount of the electricity flows in the switching element 103 which is used when performing the PWM control, the size of the element 103 is increased so as to lower the on-resistance. However, this causes an increase of the gate capacitance since the element 103 size becomes large.
The loss of the switching regulator in a case where the electric current is supplied to the load (hereinafter, referred to as the load electric current) consists mostly of loss owing to the on-resistance of the switching element 103. When the load electric current is small, the loss of the switching regulator consists mostly of a loss owing to the charging and discharging of the gate capacitance of the switching element 103.
Accordingly, the switching element 104 is reduced in size to keep the gate capacitance small even though the on-resistance of the switching element 104 is large. The efficiency of the switching regulator is improved thereby.
Japanese Laid-Open Patent Publication No. 2002-300774 discloses such a switching regulator.
However, there is a problem of generating overshoot in the output voltage as shown in FIG. 6, when the output voltage of the switching regulator is enabled to change from the low voltage to the high voltage. In addition, there is another problem that the overshoot becomes larger when the switching element with a high on-resistance is changed to a switching element with a low on-resistance at the same time as changing the output voltage.
Moreover, in the light-load drive mode in which the load electric current is small, the load circuits such as a CPU using the switching regulator 100 as a power supply for operation often stops the operation, that is, the load circuits are in the sleep mode or in the stand-by mode. In such a light-load drive mode, the operating voltage of the load circuits can often be smaller than the operation voltage in the normal drive mode. Accordingly, it is normal that the output voltage of the switching regulator is lowered so as to lower the load electric current.
However, in a case of shifting the drive mode from the light-load drive mode to the normal drive mode, when switching the control mode of the switching regulator from the PFM control to the PWM control and simultaneously changing the output voltage from the low voltage to the high voltage, the overshoot voltage is generated in the output voltage as described above. Accordingly, there is a risk of causing failure in the CPU and the other circuits.